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MANCHESTER — Massachusetts Undersecretary for Homeland Security Kurt Schwartz was on the scene of the 2013 Boston Marathon bombings within four minutes of the pressure cooker explosions that killed three spectators and injured another 264 people.

The devices, made with black powder purchased in New Hampshire, were shrapnel bombs filled with metal screws and ball bearings, said Schwartz who on April 15, 2013, was the incident commander in the Multi-Agency Coordination Center for the race.

The scene, he said, was chaotic and noisy and one of the first people he saw was Jeff Bauman, 27, of Chelmsford, Mass., whose legs were blown off.

Bauman, from his hospital bed, later would provide authorities with a description of the bombers, who police said were Chechen brothers Tamerlan Tsarnaev, 26, and Dzhokhar Tsarnaev, then 19.

Schwartz made the comments Wednesday in a talk titled "Marathon to Manhunt: The Boston Marathon Bombings as told by the Incident Commander" he gave to several hundred first responders and others attending the10th New Hampshire Emergency Preparedness Conference Wednesday at the Radisson Hotel.

He said 40 physicians and 400 nurses and other medical personnel were already at the finish line, having been stationed there in a 400-bed medical tent - essentially a field hospital - to handle exhausted and ill runners and spectators. The medical team, along with hundreds of civilians, immediately went to help the injured.

Amid the chaos, Schwartz said he could hear urgent calls for more ambulances from the two police radios he carried. Thirty were already at the scene for the marathon itself, but another 60 were called in from other communities.

Police cruisers, buses and private vehicles were used to take the injured to one of the seven Level 1 trauma hospitals in Boston.

Every one of the ambulances, as well as police cruisers, buses and private cars, were used to take the injured to one of the seven Level 1 trauma hospitals in Boston.

Schwartz said not one person who was transported died, although he said there were many who had to undergo amputations, including one person just last week.

Complicating the investigation at the scene were the hundreds of backpacks abandoned by spectators who immediately fled when the two bombs exploded, 12 seconds apart on Boylston Street, above the subway.

Schwartz said initially investigators thought there were other backpack bombs since many were covered with black powder. They toyed with the idea of shutting down the subway as the hunt for the bombers began, but decided not to because it would mean stranding about a million people.

Schwartz became a member of the Unified Command Group, which oversaw transporting the injured to hospitals, providing medical care and transportation for the 7,000 stranded runners, many of whom were dropping in the street because of cramping after running more than 20 miles, and the massive manhunt for the terrorists that resulted in an unprecedented "shelter in place" of 1.5 million people in the greater Boston area.

Authorities said the brothers ultimately were responsible for the shooting death of an MIT police officer during a Cambridge, Mass. store robbery and the wounding of a transit police officer in a police shootout and car chase during which the Chechen brothers allegedly tossed explosives at pursuing officers.

Tamerlan Tsarnaev was shot and died after his younger brother ran over him with an SUV and escaped, according to police.

Four days after the marathon bombings, police still hadn't found Dzhokhar Tsarnaev and didn't know if others were involved in the bombings, Schwartz explained.

By then, Shwartz said 2,500 police officers - many who were not asked to come - converged on a 20-block area of Watertown, Mass. searching for the then unknown terrorist. With the odds of actually finding him at 50/50, the command group came to the conclusion that Boston residents should be ordered to stay inside and that the subway system should not open.

The question became, however, who actually had the authority to make that decision. Boston Mayor Tom Menino objected.

"Not on your life. I will not shut down the subway. I will not shut down my city," Schwartz recalled him saying. An hour later, he changed his mind and told Schwartz to go out and make the announcement.

As a result, people living in suburban Watertown, seven communities abutting it and all of Boston - 1.5 million people in all - were told to stay home. Schwartz showed a photograph of Boston from mid-morning that day, the roads empty of any traffic.

By Friday afternoon, people were becoming restless. Police still hadn't found Dzhokhar Tsarnaev. Schwartz said his biggest priority at that time was getting portable toilets into the 20-block area in Watertown to meet the officers' needs, which drew laughter from the audience.

Then President Obama telephoned Gov. Deval Patrick and told him he couldn't keep Boston closed down. The Governor said he understood, hung up the phone and said to Schwartz, "You know, you can't keep Boston closed."

And so the "shelter-in-place" order was lifted and a Watertown resident went outside his house, saw blood on his covered boat, lifted the tarp, saw a wounded Dzhokhar Tsarnaev inside and called police.

Schwartz, who also was coordinator of safety and security planning for the 2014 Boston Marathon, said one thing he would do differently in the event something like the bombings were to happen again is managing the media.

The command group decided not to have news briefings unless there was something to report. He said as a result, the media made up things, such as reporting someone was under arrest, when no one was, and identifying a man in a similar baseball hat as the bomber, which was not true.

Instead, he said, he believes they should have designated someone a spokesman and had hourly briefings for the media, which grew from 500 within a half-hour of the bombings to more than 1,500 by the time the crisis was over, even if there was nothing to report.